I do think the suggestion that Tolkien "plagiarizes" from himself holds water.

Glorfindel makes an appearance in LotR. It is a bit weird, because it could have been any old elf warrior of renown -- he is not actually important to the tale, but Tolkien picked Glorfindel because...he liked Glorfindel. At this point, it has been rationalized as a blessed return, but I doubt that was Tolkien's original thinking. He put in Glorfindel because he wanted to.